The State of Silurian Stratigraphy

T H E S T A T E O F S IL U R IA N S T R A T IG R A P H Y by C .H . H o l la n d T h is sh o r t a r t ic le r e v ie w s r e c e n t d e c is io n s tak en bv th e IU G S S ub co m m issio n on Silurian Stratigraphy, 丈h e procedures by w h ic h r e c o m m e n d a tio n s a r e m ade and som e o厂the challenges sti11 加cinq workers on t八e S ilu r ia n . introdu ction The geological sciences dem and close inter n ational cooper- For stages w ithin these tw o series, the Subcom m ission ation as w ell as a rigorous and clearly understood language of agreed to use the "Sheinw oodian" and "H orrierian" stages w ords. N ow here is this m ore keenly felt than in stratigraphy for the W enlock. These again had been defined by Bassett w here, despite the luxury of long philosophical discussions on e t al. (1975). The boundary stratotype for the base of the rocks versus tim e, m any feel a sense of urgency about fo rm er w a s to be the sam e a s th a t for the W e n loc k S erie s. attem pts to achieve a standard global chronostratigraphy. A boundary stratotype for the base of the H om erian Stage The now w ell-know n pioneer w ork of the international com - w as accepted w ithin the W enlock area. m ittee on the Silu ria n- D evonian Boundary took 12 years to In the case of the Ludlow Series, H olland et al. (1963) had com plete, though th is w a s a p io n e e r in g e x e rc ise . C o r re l- nt in n s abo u t this le v e l w e r e a ls o fo u n d t o h a v e b e e n in su c h previously defined four stages, but the first and second of these w ere difficult to separate outside the W elsh Borderland grievous error that m uch readjustm ent and eventualc o 川 p r o - and the third (Leintw ardinian) stage corresponded to only one m ise w e re n eed e d before agreem ent could be re a ch e d graptolite biozone, w idespread though that is. A ccordingly, (Boucek, of al., 1966; H olland, 1965; M artinsson, 1977). N ow the Subcom m ission decided that only tw o stages w ere approp- the lU G S C om m ission on Stratigraphy, thanks especially to riate for the L udlow and, as in the case of the W enlock, these the vision and initiative of past C haIrm an A cadem ician V.V . m enner of the Soviet U nion, acts as an um brella to a w hole correspond broadly to tw o brachiopod faunas as w ell as being tied m ore precisely to the graptolite biozonal sequence (Fig. list of Subcom m issions for the various geolog ica l system s and 2). M any other groups are of course recognized as actually or of w orking groups concerned w ith settling the boundaries. potentially useful in correlation. The Subcom m ission on Silurian Stratigraphy w as constituted T hose concerned m ost directly w ith w ork in the Ludlow area from an a生垫签 body at a m eeting held in the U niversity of w ere asked to suggest nam es for these tw o new stages and Birm ingham , U .K ., in 1974. A t a form al m eeting of the the Subcom m ission approved their choice of "G orstian" and Subcom m ission held during the International G eological C on- "Ludfordian" (H olland et al. 1980). The boundary stratotype gress in Sydney, A ustralia, in A ugust 1976, it w as agreed to for the base of the G orstian Stage is the sam e as that for the proceed w ith an eight-year program designed to solve the b ase o f the L u d lo w Se ries . F o r th e ba se of the L ud fo rd ian m ajor problem s of Silurian chronostratigraphy. W ork contin- Stage, also sited within the Ludlow area (Fig. I），the decision ued using questionnaires and e ventually there w as alm ost is to use the boundary proposed by Holland et al．(1963) for unanim ous support for a com prehensive field m eeting in the base of their Leintw ardinian Stage. Britain in 1979. This m eeting ranged frorn South W ales, through the W elsh Borderland, N orth W ales, and the L ake T hese decisions w ere presented to the C om m ission on Stra- D istrict, to M offat in the Southern U plands of Scotland. tigraphy at its m eeting in Paris in A ugust 1980. Professor Various form al m eetings of the Subcom m ission w ere held, A nders M artinsson and D r. M .G . Bassett, respectively C hair- including shared m eetings w ith the W orking G roup on the m an and Secretary of the C om m ission, then circulated docu- O rdovician-Silurian Boundary since these tw o groups had a m entation and arranged a postal vote by all m em bers of the num ber of m em bers in com m on. The type areas of the C om m ission. The decisions already outlined above w ere I-landovery, W enlock, and Lud low series w ere exam ined, as ratified w ithout dissent (see M artinsson et al., 1981). w as C harles Lapw orth's fam ous locality for the O rdovician- Siluria n tra nsitio n at D o bb 's L in n n ea r M o ffa t. C ocks, Toghill, and Z iegler (1970) had already divided the Llandovery Series in to four stages, equated respectively w ith T he Su bc o m m issio n ha s 16 T itu la r M e m be rs an d a co n - the m uch used lettered and num bered divisions A l一A 4, B I-B 3, siderably larger num ber of C orresponding M em bers. T hose C l-C 3. and C }-C 6. These w ere given geographical nam es present at individual m eetings took part in discussions and based on localities w ithin the type area near Llandovery in decision m aking, but form al proposals w ere Put to postal vote South W ales. As the low est A l division at Liandovery proved by the Titular M em bers. D ecisions achieved by a substantial to be unfossiliferous and of uncertain extent, these authors m a)ority w ere : chose Dobb's L inn for the definition of the base of their first, T he sec o nd a nd th ird o f the fo u r series into w h ic h the R huddanian Stage. The basal boundaries of the rem aining Silurian System is to be divided w ere to be called ”W enlock Idw ian, Fronian, and Telychian stages w ere referred to local- Series" and "Ludlow Series", and boundary stratotypes for ities w ithin the Llandovery area. U nfortunately, detailed the ba se o f e a ch o f th ese w e re c h osen in the W elsh faunal c hanges t hrough c ontinuous boundary stratotype Borderland, w here fully described standard areas provide a sections w ith accurately defined m arker po ints w ere not setting for these sections. These are indicated in Figure I available and indeed there w as m uch discussion by Subcom - and details of the decisions w ere reported in 兰ethaia m ission m em bers on the difficulties of discrim inating be- (H olland 1980). F ull descriptions of the areas and sections tw een at least som e of these stages elsew here in the w orld. are to be found in Bassett et al. (1975) for the W enlock, A Subcom m ission w orking group is now looking carefully at and H olland et al. (1963) for the Ludlow . The sections are the so-called "northern area" of the Liandovery district, now cared for by the N ature C onservancy. w here m ore graptolites m ay be available. But w hatever the EPISO D ES, V ol. 1982, N o. 3. 2 1 N ‘ ．｜｜ k5 10 D obb's Linn and A nticosti for this purpose. They are con- －t ｜ cerned also w ith the difficult question of the biostratigraphi- ｜｜ cal horizon best chosen for the boundary. ｜ e a ｜ The fourth. post-Ludlow pre-G edinnian series of the Silurian System is the division w hich w as recognized as a result of the w ork of the Silurian-D evonian Boundary C om m ittee (M artinsson, 1977). E rrors in previous correlations had been such that the rocks deposited in this interval had actually been "lost", m ainly because of the assum ption that graptoloids m ust in som e (m ystical) sense be absent fro m post-Silurian 夕’健 2 strata.

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